The present invention is generally directed to the manufacture of integrated circuits, and more particularly is concerned with the fabrication of photomasks that are employed during such manufacture.
In the manufacture of integrated circuits, the selective diffusion of impurities into the layers of a silicon wafer, as well as the selective removal of portions of various layers, are controlled by means of a photolithographic process. For example, as shown in FIG. 1, when an impurity is to be diffused into predefined areas of a silicon substrate 10 to form components of a device such as a transistor, the substrate is first covered with a thin layer 12 of silicon dioxide (step a). The silicon dioxide is then coated with a photoresist material 14 (step b) which is selectively exposed to ultraviolet light through a patterned photomask 16 (step c). The pattern on the photomask relates to the predefined areas of the silicon into which the impurity is to be diffused.
By nature of the selective exposure to the ultraviolet light, certain portions of the photoresist layer become polymerized while other portions remain unpolymerized. The unpolymerized portions can be washed away in a suitable developing solution, to expose some of the area of the underlying silicon dioxide layer (step d). The wafer is then subjected to an etching solution which removes the exposed portions of the silicon dioxide layer down to the silicon (step e). The photoresist is removed and a silicon dioxide mask remains on the surface of the silicon (step f). The impurity is then diffused into the exposed portions of the silicon and the silicon dioxide mask can subsequently be removed in preparation for the following steps in the fabrication of the circuit.
The present invention is particularly concerned with the photomask 16 that is used to selectively expose the photoresist material on the silicon dioxide layer. In the past, photomasks have generally comprised glass plates that are covered with a photosensitive emulsion. However, the emulsion layer by itself is susceptible to scratches and tears. It will be appreciated that a scratch in an emulsion layer will enable light to pass through the photomask in an area where light should be blocked, and thus could result in an impurity being diffused into the silicon in such a manner that the resulting circuit would be defective. Accordingly, in order to provide greater durability to the photomask, a thin film of chromium has been substituted for the photosensitive emulsion. Basically, a photomask "blank" is first produced by sputtering chromium onto a glass plate. The blank is then covered with a photosensitive emulsion which is exposed to the desired pattern, and etched in a wet chemical process to remove the chromium in the areas in which it is desirable to let light pass through the mask.
The sputtering of chromium onto the glass is a vacuum based process which is expensive because of the need for surface preparation of the glass and the need for relatively low vacuum (typically on the order of 10.sup.-3 Torr). Furthermore, the pumping down of a sputtering chamber to obtain the vacuum results in the generation of contaminating particles that can be deposited on the glass surface. These particles interfere with the uniform coating of the chromium on the glass during the sputtering process, and can result in pinhole defects in the chromium layer. As with scratches in a photosensitive emulsion layer, pinholes in the chromium layer can result in the production of a defective integrated circuit, and therefore exhaustive inspection of the photomask blank is required. In addition, since chromium has a relatively high specular reflectivity, i.e., it reflects light well, an anti-reflective layer is often provided between the chromium and the glass plate in order to inhibit unwanted reflections that could degrade the resolution of the light pattern that is transmitted through the mask.